It is known in the art to determine semiconductor specimen purity by examining minority carrier lifetime using laser-induced microwave-detected photoconductive decay measurement systems (".mu.-PCD"). One such system is disclosed in U.S. Pat. No. 4,704,576 to Tributsch, et al. (Nov. 3, 1987).
In a .mu.-PCD system, the specimen is exposed to a microwave field and subjected to bombardment by a pulsed laser source. The laser's photon energy induces recombination in the specimen by creating excess charge carriers, e.g., pairs of mobile electrons and holes having excess concentrations .DELTA.n and .DELTA.p.
These carriers can affect the microwave energy reflected by the free electrons and holes in the specimen's crystal structure in a time-dependent manner. More specifically, the excess carriers increase specimen conductivity by .DELTA..sigma.=q(.mu..sub.n .DELTA.n+.mu..sub.p .DELTA.p), where q is the electron charge, and .mu..sub.n, .mu..sub.p are respectively electron and hole mobilities. Excess carrier concentrations .DELTA.n and .DELTA.p decay over time as the carriers become trapped in defects or recombine along defects within the specimen.
By detecting microwave energy reflected from the specimen, recombination decay times can be measured to determine the recombination time constant of excess carriers within the specimen. Excess conductivity (.DELTA..sigma.) measurements are indicative of defects and impurities within the specimen's crystal structure that affect the excess charge carriers.
In the simplest case, recombination is exponential with a reciprocal decay time (1/.tau.) proportional to the concentration of recombination centers, or impurities. Thus 1/.tau. (e.g., recombination lifetime) is a measure of the specimen quality.
Unless otherwise treated, the surface of a Si lattice presents many dangling bonds that can act as surface recombination centers and shorten excess carrier recombination lifetime. Because .mu.-PCD and eddy current techniques seek to measure bulk recombination and not the more rapidly occurring surface recombination, measurements typically have been made on a Si surface that is treated by thermal oxidation or chemical passivation. A significant deficiency of .mu.-PCD and eddy current measurements on non-oxidized wafers is that bulk lifetime cannot even be estimated if bulk lifetime is too large.
It would be advantageous to make stable .mu.-PCD or eddy current measurements on non-oxidized specimens for several reasons. Such measurements would avoid furnace contamination of the specimen associated with thermal oxide growing, and would avoid the problems associated with producing high quality stable, oxidized Si surfaces. Further, it is important to characterize "as-grown" (e.g., non-oxidized) Si wafers without high temperature alteration of the original distribution of any contaminants and/or the nature of crystal defects.
One known technique for chemical passivation of Si has been to initially provide an HF acid etch and then use various acid solutions. E. Yablonovitch, et al. report in Phys. Rev. Letters 57 (1986) 249 that this alternative to thermal oxidation produces an ideal (electronically inactive) surface with very low surface recombination velocity (S). In fact, this reference reports surface recombination velocities as low as about 0.25 cm/sec, and states that these values were apparently the lowest value ever reported for any semiconductor. Unfortunately, Yablonovitch, et al.'s passivation technique is limited to measuring carrier-density decay on individual points in the chemically passivated wafer.
Notwithstanding the early success of E. Yablonovitch, et al., what is needed is a method of chemically passivating a Si specimen that significantly decreases surface recombination velocity (S). What is needed is a passivation method that permits mapping bulk lifetime for an entire area, rather than mapping only individual lattice points. Preferably such passivation technique should approximate the perfection of an Si-SiO.sub.2 interface, and permit reliable in-situ bulk lifetime measurement on as-grown Si specimens. The present invention discloses a method and apparatus for accomplishing such chemical passivation.